1. Field of the Invention
The present invention relates to a technique for preventing the deterioration in display quality of a liquid crystal display device.
2. Description of the Related Art
Liquid crystal display devices are in wide use for display devices of notebook personal computers and desktop personal computers since their power consumption is small and they require only small installation space. In recent years, liquid crystal display devices for television and liquid crystal display devices for hand-held terminals such as cellular phones have been developed. Further, personal computers usable for viewing television broadcasting are being developed.
Due to diversity of products using liquid crystal devices, adaptability to various frame frequencies and horizontal frequencies are demanded of the liquid crystal devices. Here, the frame frequency indicates the display speed of a screen and corresponds to the display cycle of one screen. The horizontal frequency indicates the display speed of a horizontal line (display line) along each scanning line and corresponds to the display cycle of the horizontal line (a cycle of a horizontal synchronous signal).
A technique of correcting, when a picture signal supplied to the liquid crystal device and the horizontal synchronous signal do not coincide with each other, the difference therebetween and correctly displaying the picture signal has been proposed (for example, Japanese Unexamined Patent Application Publication No. Hei 5-46118).
Generally, driving signals to drive a scanning line and a data line of a liquid crystal panel is generated in synchronization with a synchronous signal (horizontal synchronous signal). Therefore, the timing of the driving signals of the liquid crystal panel varies in accordance with the variation in the cycle of the horizontal synchronous signal, which varies the writing time of the picture signal. Especially, the reduction in the cycle of the synchronous signal results in lack of the writing time to deteriorate display quality. Specifically, the reduction in the cycle of the synchronous signal results in lack of a timing margin, to the picture signal, of a gate clock signal for driving the scanning line and of a latch pulse signal for driving the data line, which causes problems such that a display area of the liquid crystal panel partly becomes dark. The same problem also arises when the cycle of the synchronous signal becomes shorter in accordance with the reduction of the frame cycle.